1. Field of the Invention
The present invention relates to position sensors in which displacement is indicated by the phase of an electrical signal.
2. Description of the Background Art
The shaft angle transducer is a fundamental component in modern control technology. By employing a mechanical coupling mechanism such as a rack and pinion or a spooled band, a shaft angle transducer can monitor linear as well as angular displacement. Linear displacement, however, can also be measured directly by differential or linear variable phase transformers, and ferromagnetic position transducers. See, e.g., Rhod Zimmerman, "Resolvers As Velocity and Position Encoding Devices," PCMI, September 1986, pp. 47-54; Don Overcash, "Selecting the Proper Position Sensor," Control Engineering, Septemper 1986, pp. 294-302.
Differential or linear variable phase transformers offer many advantages such as infinite resolution, ruggedness, input/output isolation, and operation over wide temperature ranges. A kind of variable phase transformer sold under the trademark "Inductosyn" is also capable of very high accuracy. As described in Tripp et al. U.S. Pat. No. 2,799,835, this kind of variable phase transformer includes two relatively moveable inductor supports. A first one of the supports carries a pair of first and second windings each in the form of a flat metallic ribbon following a sinuous path along the direction of relative displacement between the two supports. The first and second windings are mounted in positional phase quadrature relation with respect to each other and are excited in electrical phase quadrature by respective sine and cosine signals. The second support carries a third winding similar to the first and second windings. The third winding is also aligned along the direction of relative displacement and is positioned for mutual coupling with the first and second windings. Therefore, the third winding provides an electrical signal having a phase indicating the relative displacement between the supports.
The "Inductosyn," however, must be used as an incremental device for sensing displacements in excess of the wavelength of the windings, because a relative displacement of one wavelength between the two supports results in the same phase indication. For some other kinds of position sensing variable phase transformers, attempts have been made to obtain accurate absolute position sensing over a relatively wide range. Pauwels et al. U.S. Pat. No. 4,282,485, for example, discloses a linear variable phase transformer employing multi-layer helical coils in which the sine and cosine driven windings have a density of windings which is a sinusoidal function of position along the length of the transformer. Shimizu et al. U.S. Pat. No. 4,604,575 discloses a rotational position detection system including a first rotary variable phase transformer detecting an absolute rotational position within a complete circumference, a second rotary variable phase transformer detecting absolute rotational position within an integral submultiple of a complete circumference, and means for combining the positions detected by the two transformers to obtain an indication of absolute rotational position.